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Reading Part:
Populations of the yellow cedar, a species of tree that is common in northwestern North America, have been steadily declining for more than a century now, since about 1880. Scientists have advanced several hypotheses explain this decline.
One hypothesis is that the yellow cedar decline may be caused by insect parasites, specifically the cedar bark beetle. This beetle is known to attack cedar trees; the beetle larvae eat the wood. There have been recorded instances of sustained beetle attacks overwhelming and killing yellow cedars, so this insect is a good candidate for the cause of the tree’s decline.
A second hypothesis attributes the decline to brown bears. Bears sometimes claw at the cedars in order to eat the tree bark, which has a high sugar content. In fact, the cedar bark can contain as much sugar as the wild berries that are a staple of the bears’ diet. Although the bears’ clawing is unlikely to destroy trees by itself, their aggressive feeding habits may critically weaken enough trees to be responsible for the decline.
The third hypothesis states that gradual changes of climate may be to blame. Over the last hundred years, the patterns of seasonal as well as day-to-day temperatures have changed in northwestern North America. These changes have affected the root systems of the yellow cedar trees: the fine surface roots now start growing in the late winter rather than in the early spring. The change in the timing of root growth may have significant consequences. Growing roots are sensitive and are therefore likely to suffer damage from partial freezing on cold winter nights. This frozen root damage may be capable of undermining the health of the whole tree, eventually killing it.

中文翻译:
黄杉是北美西北部常见的树木。但是从1880年开始的一个多世纪以来,黄杉的数量在逐渐减少。目前,科学家提出了几个假说来解释黄杉数量的下降。
其中一个假说认为黄杉数量下降是由以衫皮甲虫为代表的寄生害虫导致。这种甲虫据了解是以杉树皮为食,而其幼虫则以木质部分为食。史上不乏对于甲虫侵蚀和导致黄杉大量死亡的记录。因为,这种甲虫很可能是黄杉数量减少的原因。
第二种假说认为是棕熊导致了黄杉数量的减少。熊有时会扒下黄杉的树皮用以食用,因为黄杉树皮还有很高的糖分。事实上,黄杉树皮的含糖量同棕熊主要使用的野生浆果相差无几。虽然棕熊扒树皮的行为不会直接导致黄杉的死亡,但是它们这种破坏性的觅食行为却因为降低了树木的生命力,而间接导致黄杉数量的减少。
第三个假说是气候的缓慢变化导致了黄杉数量的减少。在过去的一百年里,在北美西北部,无论是季节性的气候更替还是每天的气温调节都发生了变化。这些变化对黄杉根系产生了影响。以前,黄杉的表面细根是初春开始生长的;而现在却在晚冬就开始生长。根系生长时间的变化可能会造成非常严重的后果。新生的根系都非常敏感,因而非常容易因为冬季夜晚部分结冰的温度受到损害。这种根系冻伤现象可能会导致整个树木健康状况下降,并逐渐导致死亡。
Listening Part:
Question:
Summarize the points made in the lecture, being sure to explain how they cast doubt on specific points made in the reading passage.
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